1. Field of the Invention
The present invention relates to liquid crystal displays, and more particularly, to repairing an open gate line in an LCD.
2. Description of the Related Art
Generally, a liquid crystal display (LCD) utilizes optical anisotropy and polarization properties of liquid crystals. The liquid crystals also have directionality properties imparted by their slender and long structure. An electric field applied to the liquid crystal can control the direction of molecular arrangement in the liquid crystal.
Accordingly, adjusting the direction of molecular arrangement modulates light polarized due to the optical anisotropy to represent an image information.
Such liquid crystals are classified according to their electrical characteristics as either a positive liquid crystal or a negative liquid crystal. A liquid crystal molecule whose permittivity anisotropy is positive has a major axis arranged in parallel with the direction of the applied electric field. On the other hand, a liquid crystal molecule whose permittivity anisotropy is negative has a major axis arranged perpendicular to the direction of the applied electric field.
Today, an active matrix LCD, in which thin film transistors coupled to pixel electrodes coupled are arranged in a matrix form, is widely used because it has an excellent resolution and is responsive enough for displaying rapidly changing images. A liquid crystal display panel is often constructed as explained below with regard to FIG. 1, which shows an exploded perspective view of a LCD.
Referring to FIG. 1, the LCD 11 includes an upper substrate 5 and a lower substrate 22, and the liquid crystal 14. The liquid crystal 14 is disposed between the upper substrate 5 and the lower substrate 22. The upper substrate 5 is provided with a color filter that includes a black matrix 6, a sub color filter 8, and a transparent common electrode 18 formed on the color filter 7. The lower substrate 22 supports circuitry for individual pixel areas P, including a pixel electrode 17 formed in each pixel area P, a thin film transistor T serving as a switching element, a data line 15, and a gate line 13.
The lower substrate 22 is also called an array substrate, and the switching thin film transistors T are arranged in a matrix form. The thin film transistors T are generally located proximate to where the gate line 13 and the data line 15 intersect. The areas bounded by the intersection of the gate lines 13 and the data lines 15 are the pixel areas P.
In operation, the gate line 13 applies a control voltage pulse that drives a gate electrode of the thin film transistor T, and the data line 15 applies a signal voltage for driving a source electrode of the thin film transistor T. When the signal voltage is applied to the source electrode and the control voltage pulse drives the gate electrode, the signal voltage is applied through the transistor to the activate the pixel. When the signal voltages applied to the remaining source electrodes are lower than the liquid crystal driving voltage, the pixels connected to the remaining source electrodes remain inactive, and only a single pixel activates.
In order to independently drive each pixel, gate lines and data lines are arranged in a matrix form over an overall display area. As noted above, the gate lines and the data lines drive the thin film transistors to activate or deactivate selected pixels.
However, defects may impair the operation of the circuitry formed on the array substrate during manufacturing. The defects are generally classified as a dot defect, a line defect, or a display spot defect according to impact that the defect has on the operation of the pixels. The dot defect is caused due to a thin film transistor failure and a pixel electrode failure, and the line defect is caused by a breakdown of the thin film transistor due to a gate or data line open circuit or short circuit, caused, for example, by static electricity.
The defects are increasingly problematic and important as the display area of the LCD increase. Particularly, even a single line defect is considered a fatal defect. The defective LCD is considered valueless because a whole row or column of pixels is rendered inoperable. In other words, the LCD is not considered a saleable product and, as a result, the resources spent to produce the defective LCD are wasted.
In the past, a line defect in the form of an open circuit in a gate line has been repaired as will be described next. In particular, FIGS. 2A, 2B, and 2C are plan views schematically illustrating a partial portion of an array substrate of a LCD, and show the prior repair technique for an open circuit defect in a gate line.
Referring to FIG. 2A, the gate lines 13 and 13′ and the data lines 15 and 15′ intersect to define a pixel area P. A neighboring pixel area P′ is also shown, and the electrodes 17 and 17′ are disposed within the pixel areas P and P′ respectively. A thin film transistor T that includes a gate electrode 31, a source electrode 33 and a drain electrode 35 is formed proximate to the intersection area of the gate lines 13 and 13′ and the data lines 15 and 15′.
In addition, the pixel electrode 17 is electrically connected to the drain electrode 35 of the thin film transistor T via a contact hole 37 provided within the pixel area P. A predetermined portion of each pixel electrode 17 and 17′ is overlapped with the gate line 13′. For example, a portion of the pixel electrodes 17 and 17′ extend over the gate line 13′, and thereby form individual capacitors with capacitance Cst.
Here, the source electrode 33 and the drain electrode 35 are formed spaced apart under the gate electrode 31 by a predetermined interval, and an active channel (not shown) formed from a semiconductor layer (not shown) for the transistor T is thereby exposed in the interval.
When a scanning pulse is applied to the gate electrode 31 of the thin film transistor T, the voltage on the gate electrode 31 increases and the thin film transistor T is turned on. At this time, a liquid crystal driving voltage is applied to the pixel electrode 17 from the data line 15 through the source electrode 33 and the drain electrode 35 of the thin film transistor T, so that a pixel capacitance is charged. The pixel capacitance is a sum of a liquid crystal capacitance Clc and the storage capacitor capacitance Cst.
In such an array substrate, as shown in FIGS. 2B and 2C, a gate open (GO) may occur due to impurities or other factors occurring during a manufacturing process. Here, the gate open (GO) represents a break or open circuit at or over a predetermined portion of the gate line 13′.
FIG. 2B illustrates a case in which the gate line 13′ has a gate open (GO) contained within the pixel area P, and more particularly contained within the extent of the pixel electrode 17. On the other hand, FIG. 2C illustrates a case in which the gate line 13′ has a gate open (GO) at a stepped portion that occurs where the gate line 13′ intersects with the data line 15′.
Referring to FIG. 2B, when the gate line 13′ has a gate open (GO) within the pixel area P, a laser welding process may be employed to repair the gate line 13′. Note that the gate line 13′ includes a first portion 20 and a second portion 21 on opposite sides on the gate open (GO). To repair the gate open (GO), the first portion 20 is connected to the pixel electrode 17 (where the first portion 20 overlaps the pixel electrode 17), and the second portion 21 is also connected to the pixel electrode 17 (where the second portion 21 overlaps the pixel electrode 17). The connection point formed by the laser is labeled “C” in FIG. 2B.
By doing so, the opened gate line 13′ incorporates the pixel electrode 17 to bypass the gate open (GO).
In this case, the corresponding pixel electrode 17 does not play operate as a conventional pixel electrode. As a result, the associated pixel is defective and has the appearance of a one pixel spot on the LCD. Nevertheless, a one pixel spot is within the guidelines for acceptance as a good LCD panel, and may therefore by sold to generate revenue.
Note, however, that this technique cannot be applied to bypass all gate open (GO) manifestations. For example, as shown in FIG. 2C, a gate open (GO) may occur in the gate line 13′ at the stepped portion where the gate line 13′ intersects with the data line 15′. In other words, since the gate line portions 24 and 25 (disposed at first and second sides of the opened gate line 13′) overlap different pixel electrodes 17 and 17′, forming connection points C between the gate line portions 24 and 25 and the underlying pixel electrodes 17 and 17′ does not fix the gate open (GO) condition.
Since the technique described above cannot repair the gate open (GO) defect shown in FIG. 2C, the corresponding LCD panel must be discarded, thereby reducing the manufacture yield of the LCD panel, and wasting valuable product manufacturing resources.
Thus, a need has long existed for addressing the gate open (GO) defects noted above, and related problems previously experienced.